1. Field of the Invention
The present invention relates generally to the heat treating process known as annealing, and, more particularly, to a convection diffuser and charge support system with fluid cooled base components for use in an annealing furnace, and to methods for carrying out an annealing process by operating a water cooled convection diffuser and charge support system in a particularly advantageous manner to provide a controlled cooling of gases that are circulated within the closed environment of the annealing furnace during an annealing process without causing collapse of the positive pressure atmosphere of the furnace.
2. Prior Art
Annealing is a heat treatment process whereby a charge of material is heated to and held at an elevated temperature for a sufficient length of time to assure that metastable conditions in the material, such as frozen-in strains, dislocations, vacancies, and the like are permitted to achieve thermodynamic equilibrium. With ferrous materials, the term "annealing" is usually used in the sense of a "full" annealing process which involves a change of phase, whereby the metal is heated into the austentic region, and thereafter cooled back to ambient temperature to develop a softened structure of pearlite and ferrite within the metal.
Where the charge of ferrous material being annealed has been cold-worked, the annealing process is used to soften the material to relieve such hardness as has been induced during cold working. Cold-working tends to increase the dislocation density of a metal manyfold. By way of example, a cold-worked piece of metal may have a dislocation density that is 10.sup.6 greater than that of an unworked specimen of the same material. Since dislocations within cold-worked metal are surrounded by strain fields, the greater the number of dislocations, the greater is the magnitude of the "free energy" which is stored in these strain fields and which can be released during annealing to furnish a driving force that will assist in bringing the dislocation density back to within a desired range.
In order to properly anneal a charge of ferrous material, it is important to confine the charge within an enclosure wherein a non-oxidizing environment is maintained. The gases which define the non-oxidizing environment must be circulated within the enclosure during annealing to assure that convection heat transfer takes place efficiently to enable the annealing process to be carried out in a reasonable period of time. Similarly, during cooling of the charge, proper gas flow is important to effect convection cooling.
Where the charge of material to be annealed takes the form of a plurality of coils of rolled steel, the enclosure utilized to surround and support the charge conventionally includes an annular base structure atop which a vertical stack of the coils of steel to be annealed is supported, with the coils positioned coaxially one atop another. The enclosure also includes a generally cylindrical shroud which cooperates with the base structure to contain the stacked charge of coils and to define an enclosed environment within which hot gases of the controlled environment are caused to circulate.
In conventional practice, a fan is disposed in a centrally located chamber or hole formed through the base structure for forcing circulation of the gases of the non-oxidizing atmosphere throughout the enclosure. The conventional flow path for circulation includes a flow of gases downwardly through the stack of coils, and upwardly along the outer surfaces of the coils. Convector plates are interposed between adjacent ones of the stacked coils to provide convection flow paths for diverting some of the circulating gases between the ends of adjacent coils. The base structure on which the lowermost coil rests is provided with vanes for directing gases discharged from the fan outwardly and upwardly about the stack of coils.
While the gas circulation passages of a newly built diffuser base may provide a gas flow pattern that is relatively effective in disbursing gases throughout the enclosure, once the newly built base has been in service for several months, its flow passages often become deformed due to thermodynamically induced stress which results in creep growth that requires trimming, with the result that the passages no longer operate as intended to properly direct gas flow.
Moreover, inasmuch as the structures which define the vanes of present day diffuser bases are traditionally formed as weldments of relatively soft steel, the vanes tend to become deformed and/or broken during use, thereby further adding to the inefficiency and unpredictability of a diffuser base after it has been in use for a significant period of time. Thus, present day diffuser bases not only fail to operate efficiently and effectively over long periods of time, but also require frequent checking for structural integrity, cleaning and repair.
While proposals have been made to utilize water cooled heat exchanger equipment of various types to expedite the cooling of the heated gases that are present in an annealing furnace enclosure after a charge of metal has been heated sufficiently to anneal it, problems have arisen in efforts to implement these proposals. In many instances, the heat exchanger equipment has been found to interfere with proper flow of the gases within the furnace enclosure. In most instances, the initiation of a flow of cooling fluid through the heat exchanger equipment has been found to cause so rapid a temperature drop in the gases within the enclosure that the positive pressure atmosphere of the gases is "collapsed" such that a negative pressure results. The tendency toward creation of a negative pressure within the furnace enclosure (the pressure is "negative" in comparison to the pressure of surrounding ambient air) has the very undesirable effect of causing ambient air to be drawn into the enclosure. With the introduction of ambient air, and the oxygen it contains, deleterious effects are encured by the charge of metal being annealed.
In order to overcome the deleterious effects of an unwanted introduction of ambient air into the annealing furnace enclosure, it has been found necessary, in implementing most prior proposals for the use of water cooled heat exchanger equipment in an annealing furnace, to initiate operation of the heat exchanger equipment while the gas temperature within the furnace enclosure is sufficiently high that the resulting collapse of the positive pressure atmosphere within the furnace (and the attendant drawing of oxygen containing ambient air into the furnace enclosure) can be overcome by re-establishing a positive pressure, non-oxygen-containing atmosphere while the charge of metal being annealed is still sufficiently hot to permit that the deleterious effect that is occasioned by the action of oxygen on the metal to be treated and overcome (i.e., reversed) before the charge of metal has cooled to an extent that the deleterious effect is retained by the treated metal.
The need to "turn on" previously proposed heat exchanger equipment while the temperature within the closed environment of an annealing furnace is relatively high causes the heat exchanger equipment to be subjected to a wrenching thermal shock that significantly diminishes the life expectancy that this equipment would otherwise enjoy if it were not necessary to initiate its cooling function while the temperature within the annealing furnace enclosure is so high. Due to the enormity of the thermal shock to which the heat exchanger equipment is subjected, its construction must be significantly strengthened and enhanced to guard against shock induced failure, whereby the physical size of this equipment and the attendant extent to which the equipment interferes with the proper flow of gases within the furnace enclosure are undesirably increased, and the overall performance of the furnace suffers.
Stated in another way, the implementation of prior proposals for the use of fluid cooled heat exchange equipment in the environment of an annealing furnace has been found to encounter more problems, and problems of greater severity, than would be expected; moreover, many of these implementation efforts have been found to fail as the result of components being subjected to excessive thermal shock, and/or to be unacceptable in view of the deleterious effect that the components have on desirable operating characteristics of an annealing furnace.
3. The Referenced Diffuser System Patent
The invention of the Diffuser System Patent addresses the foregoing and other drawbacks of the prior art by providing a durable, novel and improved convection diffuser and charge support system. In preferred practice, the invention of the Diffuser System Patent utilizes charge support system components that are formed from highly durable, shock resistant nodular cast iron, with selected ones of the components incorporating cast-in-situ cooling conduits.
In accordance with the preferred practice of the invention of the Diffuser System Patent, a diffuser base, a base-encircling ring, and a plurality of charge-support convector plates are all formed as castings of a particularly durable material known as nodular iron. Nodular iron is cast iron which has been treated while in a molten state with an alloy that contains an element such as magnesium which favors the formation of spheroidal graphite when the cast iron solidifies, whereby the resulting product is more ductile and durable than normal cast iron.
The casting or castings of the diffuser base define a set of horizontally extending gas circulation passages that are shielded from above by an integrally formed overlying top wall. The top wall also serves to strengthen a plurality of upstanding gas directing vanes that are formed as integral parts of the diffuser base casting or castings, whereby there is much less tendency for deformation and breakage of the vanes. The casting or castings which form the base-encircling ring define an array of curved, upwardly opening passages that cooperate with the primary flow passages of the diffuser base to direct the gases of the non-oxidizing atmosphere along particularly advantageous, substantially helical flow paths about the periphery of the stack of coils.
By forming the base-encircling ring as a structure which is separate and apart from the diffuser base, the base and the ring are easily separated one from the other for occasional cleaning. Moreover, this feature of separability enables the primary gas flow passages formed in the diffuser base to extend almost entirely horizontally and to thereby be shielded from above by the top wall of the base to prevent debris from falling into these carefully configured passages. The passages formed in the base-encircling ring comprise, in effect, upwardly curved extensions of the horizontally extending primary flow passages formed in the base. When the diffuser base is separated from the ring, the passages of both of these structures are rendered readily accessible for cleaning and maintenance.
Other features of the invention of the Diffuser System Patent lie in the optional use of (1) one or more cast-in-situ cooling conduits provided in the base-encircling ring, and (2) a continuous, depending skirt wall that is formed as an integral part of the base-encircling ring. The cooling conduit or conduits may be utilized during the cool-down part of an annealing cycle to assist in cooling such gases as are circulated within the controlled environment. The depending skirt extends into an upwardly facing annular groove that is conventionally provided in the furnace base. The skirt engages a fibrous refractory sealant positioned in the groove and thereby assists in effecting a gas-tight seal that prevents ambient air from entering the closed controlled environment of the annealing furnace. The skirt also shields the surrounded portion of the furnace from deterioration.